Natural isolates of ECHO virus type 25 with extensive variations in IRES sequences and different translational efficiencies.

The complete nucleotide sequence of the 5'-untranslated region (5'-UTR) of the genome of three ECHO virus type 25 strains (the JV4 reference strain and two wild isolates) was determined. The 5'-UTR of the two isolates shared 85 and 82% nucleotide identity with the reference strain. The overall folding of the predicted secondary structure of the ECHO virus 5'-UTRs showed significant conservation with that of the poliovirus. Most significant differences were observed in specific domains of the predicted IRES of the wild isolates. The efficiency with which ECHO virus type 25 5'-UTRs promoted internal initiation of translation was examined in an in vitro translation system. At low doses of input RNA, all three 5'-UTRs were similarly efficient in promoting internal ribosome entry and behaved similarly to the poliovirus IRES in that downstream cistron translation was markedly stimulated by the addition of HeLa cell extracts to standard reticulocyte lysates. At high RNA doses, the JV4 5'-UTR was much more efficient than the other two ECHO virus 5'-UTRs. This difference was much less marked when reticulocyte lysates were supplemented with 20% HeLa S-10 extracts, which suggests that the two less efficient 5'-UTRs were characterized by a reduced binding capacity to at least one factor present in HeLa cells. In MRC5 cell S-10 extract, internal initiation of translation was stimulated from the IRES of all three ECHO strains, the M1262 IRES being the least stimulated.